Manufacture of wood fiber hardboard



Aprll 7,1959 B. o. STEWART 2,380,655

' MANUFACTURE OF WOOD FIBER HARDBOARD Filed July 24, 1957 A HIGH TEMPERATURE STEAM TREATMENT OF WOOD CHIPS REDUCE WOOD CHIPS SUBSTANTIALLY TO FIBERS FORM WET MAT OF FIBERS ON SCREEN .sQuEEzE-PREss TO REDUCE MOISTURE CONTENT DRY IN HOT AIR CONTINUOUS DRYER REMovE THIN LAYER FROM EACH FACE HOT PRESS TO CONSOLIDATE INTO HARDBOARD INVENTOR. Basil O. Srewurr BYWZ) ATTORNEY United States Patent 2,880,655 MANUFACTURE OF WOOD FIBER HARDBOARD Basil 0. Stewart, Kenmore, N.Y., assignor to National Gypsum Company, Buffalo, N.Y., a corporation of Delaware Application July 24, 1957, Serial No. 673,791 12 Claims. (CI. 92-61) This invention relates to the manufacture of heat and pressure consolidated wood fiber hardboard. More particularly, it is directed to a process for the preparation of a dried, wet-formed mat of wood fiber prior to the consolidation thereof by combined heat and pressure.

The manufacture of wood fiber hardboard consists of the formation of a relatively porous mat of woody material and the consolidation of the mat by combined heat and pressure. A high pressure steam treatment of the woody material prior to forming, providing to some degree a conversion of the lignins and the cellulose, is commonly preferred in order to obtain, when subsequently formed and consolidated, a superior hardboard, and it is with regard to this method of hardboard manufacture that the present invention is concerned.

Of particular concern, one result of the steam treatment of woody material is the formation of water-soluble sugary material. Following the mat formation and drying, the water-solubles are, in large part, ultimately deposited on the wood fibers of the mat. The usual manner of drying, whether or not it is preceded by passing the mat through squeeze rolls to substantially reduce the water content, will include the subjection of the mat to circulating, relatively dry, heated air, as in a kiln. The mat is reduced in moisture content to a bone-dry condition or at least to a moisture content of less than preparatory to consolidation by heat and pressure to a relatively thinner hardboard of a specific gravity, ordinarily about 1.0 or higher.

In prior processes, generally as described above, it has been considered essential to remove, in some manner, a. substantial proportion of the water solubles prior to the final drying, since it is known that their presence creates a problem of the surface sticking to the high temperature and pressure consolidating means, resulting in delami' nated, unacceptable end products. This has been accomplished heretofore by additional washing steps at some point in the process prior to drying, wherein the solubles are carried away in the wash water. The washing steps are unduly costly, and, of greater significance, create a very serious problem of waste disposal.

It is an object of the present invention to provide a. process for making hardboard generally in accordance with the above described preferred steps, wherein the sticking caused by the solubles is avoided, without requiring the costly and undesirable additional washing steps' for their removal, and further permitting a more rapid high temperature consolidation.

A further object of the invention is to provide a process of making hardboard wherein a high percentage of all. of the natural woody material is retained and employed. within the final product.

A still further object of the invention is the avoidance in the manufacture of hardboard of any substantial. amount of waste disposal and the consequent stream pollution problems, and providing instead a recovery of sludge "ice and solubles in a form readily adaptable for further advantageous use.

A still further object of the invention is to provide an improved process of hardboard manufacture wherein a hardboard is produced having improved homogeneous character, improved warp resistance and lighter and evener color.

An even further object is to provide a hardboard of improved receptability for painting and decorating.

These and other objects and advantages will appear more fully when considered in connection with the following detailed description of a preferred embodiment of the invention and the accompanying drawing in which a flow sheet of the process of the invention is set forth.

A preferred form of the process of the present inven-' tion, as shown in the drawing, consists of reducing raw wood to chips of a size in the order of A" x across the grain and with the grain. The chips are then subjected to a high temperature steam treatment, to chemi: cally convert some of the ingredients therein, principally the lignins, into a form whereby they will subsequently provide a binder material in the consolidated board, this being in accordance with many similar prior processes. As is known, there is also produced by this treatment, a certain amount of water soluble matter, such as wood sugars, which have long been recognized as undesirable, but unavoidable.

The steam treated chips are then reduced in size substantially to fibers by any known means such as by defibrators having closely opposed, oppositely rotating discs. The fibrous material, dispersed in an aqueous slurry, is then formed into mats by known screen forming means, such as a Fourdrinier machine.

The mats, still consisting of considerable water, are then squeezed, such as by press rolls, to substantially reduce the water content, and are then air dried, as in a hot air continuous dryer, to a water content of from 0% to 10%, preferably as close to 0% as practical.

In accordance with the present invention, a thin layer of surface material is then removed from the top and surfaces of the mat, the total material removed in the preferred form of the invention amounting to not necessarily more than 2%, and depending principally on the smoothness or flatness of the surface and the means employed for performing the removal. When the surfaces are substantially smooth prior to the removal of surface material, the material removed may be kept to a minimum, and the material so removed may be limited to a thin substantially non-fibrous layer of the above discussed undesirable water soluble matter. It will be realized that any greater amounts of material could be removed in accordance with the invention but that removal of excessive amounts would be merely unnecessary and, thus, costly.

Example I In a preferred form of the invention, the substantiallydry mats are subjected to a surface brushing step. usingv a relatively stiff, rotary wire brush, having an axial length at least equal to the width of the mat. Each face of the mat is successively brushed or alternatively both faces. may be simultaneously brushed. The proximity of the brush to the mat is adjusted to provide the desired amount of removal of surface material by the brushing action, ordinarily less than 1% material removal from each face being sufficient in accordance with the invention.

The surface material so removed is collected by suc-' tion ducts and may be readded to successive mats prior to screen forming, chemicaly converted to other suitably by-product form, or disposed of as desired.

The surface brushed mat is subsequently consolidated .by the usual high temperature and pressure process with? out encountering a harmful degree of sticking.

Example II In accordance with a second embodiment of the invention, the substantially dry mats are subjected to a surface planing action using known horizontal planing machines with planing cutters of a length preferably at least equal to the width of the mat. Both faces of the mat are planed to remove preferably less than 1% of the mat material from each face. Material removed may be handled as in Example I.

The surface planed mat is subsequently consolidated by the usual high temperature and pressure process without harmful sticking.

Example III In a process otherwise similar to Examples 1 and II, the substantially dry mats may be subjected to a surface abrading action, such as with abrasive wheels or belts, providing a suitable removal of surface material from each face thereof.

Further modifications of the process of the invention would consist of sawing, milling or otherwise removing the thin surface layer of the dried mat prior to heat and pressure consolidation.

The physical removal of the thin surface layer of the mat has been found to remove a very considerable percentage of the undesirable water solubles present within the mat, which have been known before to be undesirable matter and which were heretofore generally removed by a costly washing of the fibrous material or mat. The success of the novel mat surface removal is apparently the result of a migration of substantially all of the Water solubles to the surface during the prior squeezing and air drying.

It has further been found that under most preferred air drying conditions, a greater percentage of water solubles tend to migrate to the mat top surface in comparison to the mat bottom surface. Accordingly, the advantages of the invention may be better realized by providing for a slightly greater removal of material from the mat top surface than from the mat bottom surface.

Following the removal of the above said surface material from the mat, the mat is subjected to a high temperature, high pressure consolidation to form a hard, dense grainless board of over 1.0 specific gravity.

The actual volume of surface material removed is relatively negligible in comparison to the volumes of water heretofore required to perform the prior washing out of solubles. Furthermore, such material as is removed in accordance with the invention, is in a form well adapted for subsequent treatment for readdition to subsequently formed mats, or otherwise for more economical disposal.

The consolidation of the mat having had the surface layer removed may be performed without any undesired sticking which would otherwise result with any substantial amounts of the water solubles on the surface. An improved hardboard is provided having less tWo-sidedness resulting in less warp and more homogeneity. The improved board is of relatively lighter color, adapted to easier decoration as by paint and is of relatively lower cost by reason of more economical waste disposal and faster consolidation.

Having completed a detailed disclosure of the preferred embodiments of my invention so that those skilled in the art may practice the same, I contemplate that variations may be made without departing from the scope of the invention as defined in the appended claims.

I claim:

1. In the manufacture of heat and pressure consolidated wood-fiber hardboard, wherein wood has been reduced substantially to fibers and said fibers have been subjected to an aqueous digestion wherein a substantial quantity of water soluble derivatives thereof are produced, the further steps comprising wet felting said fibers to form a porous wood-fiber mat containing a substantial proportion of said water soluble derivatives, drying the mat to a moisture content of less than 10%, effecting said drying of said mat in a manner whereby a substantial proportion of said water soluble derivatives are caused to move to a thin outer skin on each face of said mat, removing said thin outer skin throughout the extent of each face of the dried mat, and consolidating said mat by suitable heat and pressure.

2. The method of claim 1, wherein the thin outer skin material removed is less than 2% of the material of said mat.

3. The method of claim 1, wherein the thin outer skin material removed therein is recycled into subsequently formed wood fiber mats prior to completion of the'wet felting thereof.

4. The method of claim 1, wherein said thin outer skin is removed by a brushing of the faces of said mat with a relatively stiff bristled brush.

5. The method of claim 1, wherein said thin outer skin is removed by planing the faces of said mat.

6. The method of claim 1, wherein said thin outer skin is removed by abrading the faces of said mat.

7. In the manufacture of heat and pressure consolidated wood fiber hardboard, wherein wood has been reduced substantially to fibers and said fibers have been subjected to an aqueous digestion wherein a substantial quantity of water soluble derivatives thereof are produced, the further steps comprising wet felting said fibers to form a porous wood-fiber mat containing a substantial proportion of said water soluble derivatives, drying the mat to a moisture content of less than 10%, effecting said drying of said mat in a manner whereby a substantial proportion of said water soluble derivatives are caused to move to a thin outer skin on each face of said mat and a relatively greater proportion of said movement is in an upward direction, removing a thin outer skin throughout the extent of the face of the dried mat which was disposed upwardly during said drying and removing a rela tively thinner outer skin throughout the extent of the face of the dried mat which was disposed downwardly during said drying, and consolidating said mat by suitable heat and pressure.

8. The method of claim 7, wherein the thin outer skin material removed is less than 2% of the material of said mat.

9. The method of claim 7, wherein the thin outer skin material removed therein is recycled into subsequently formed wood fiber mats prior to completion of the wet felting thereof.

10. The method of claim 7, wherein said thin outer skin is removed by a brushing of the faces of said mat with a relatively stiff bristled. brush.

11. The method of claim 7, wherein said thin outer skin is removed by planing the faces of said mat.

12. The method of claim 7, wherein said thin outer skin is removed by abrading the faces of said mat.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN THE MANUFACTURE OF HEAT AND PRESSURE CONSOLIDATED WOOD-FIBER HARDBORAD, WHEREIN WOOD HAS BEEN REDUCED SUBSTANTIALLY TO FIBERS AND SAID FIBERS HAVE BEEN SUBJECTED TO AN AQUEOUS DIGESTION WHEREIN A SUBSTANTIAL QUANTITY OF WATER SOLUBLE-DERIVATIVES THEREOF ARE PRODUCED, THE FURTHER STEPS COMPRISING WET FELTING SAID FIBERS TO FORM A POROUS WOOD-FIBER MAT CONTAINING A SUBSTANTIAL PROPORTION OF SAID WATER SOLUBLE DERIVATIVES, DRYING THE MAT TO A MOISTURE CONTENT OF LESS THAN 10%, EFFECTING SAID 